This invention relates to circuit arrangements for switching transformer taps under load carrying conditions, and in particular to circuit arrangements of this type which are applicable to the type of motor speed control used with wound-rotor motors.
The Scherbius drive and the Kramer drive systems are commonly used for variable-speed motor control. Solid state Scherbius or Kramer drives control the speed of a wound-rotor motor by controlling the rotor current and at the same time returning the slip power, which would otherwise be wasted, to the primary power supply system. By regulating the current in the rotor winding, these systems control the motor torque and thus the motor speed.
It is known that the voltage induced in the rotor of an induction motor is maximum at start-up when the motor speed is zero, and decreases as the rotor speed increases. For example at 94% speed, the rotor voltage is only 6% of the maximum, or starting value. The control system must therefore handle a wide range of voltage.
Practical systems have been designed which reduce this voltage range in order to reduce the size and cost of the control system. One such arrangement employs an autotransformer to decrease the rotor voltage supplied to the control system at low rotor speeds and to increase the rotor voltage supplied to the control system at high rotor speeds.
The control system is connected to a low voltage tap of the autotransformer for operation from zero rotor speed to 70% rotor speed, for example. When the rotor reaches 70% speed and the rotor voltage has decreased to 30% of its initial, or zero speed, value, the control system is switched to a high voltage tap of the autotransformer to increase the voltage into the control system.
However, in known systems, the voltage on the autotransformer must be reduced to zero at the time of switching from the low voltage tap to the high voltage tap because it is not practical to attempt to switch the high currents involved, and also, severe transients would result. Consequently, during the switching period, power is disconnected from the motor for an interval of one to two seconds during which time the motor speed will decrease. During much of this interval, the motor torque is less than that required to hold the load. In an application for controlling the speed of a coolant pump motor in a nuclear reactor, for example, the total inertia is primarily that of the motor. In such application a speed drop of several percent can be expected during the switching interval. Thus, motor controllers which require the removal of power from the transformer during tap switching would be unsuitable in this application, or other applications which require a stepless speed control characteristic over the required speed range.